1999 Jeep Grand Cherokee vibration and whining/humming noise

My jeep has developed a whining/humming noise that only occurs during acceleration. The sound is vehicle speed dependent (the sound pitch gets higher) not engine speed dependent. On flat ground it isnt too bad unless I press the gas fairly hard but when at the top of a hill where the hill starts to level off, the vibration and sound is awful. Please help me locate the source and an idea of a repair cost.